1. Field of the Invention
The present invention relates to a flexible printed circuit board and a circuit-board connection structure in which a flexible printed circuit board is solder-mounted on a printed circuit board.
2. Description of the Related Art
In an optical transmission/reception module having a flexible printed circuit (FPC) board as an electrical interface, a high-speed signal at tens of Gbps is transmitted through a connection portion between a printed circuit board and the FPC board. As the FPC board, typically, a two-layer FPC board in which a base material is made of polyimide and which has a microstrip line structure composed of a transmission line and a ground layer is used. The transmission line is configured as a single-phase line or differential lines. To connect the FPC board and a printed circuit board, direct coupling by soldering is used.
In a technique described in Japanese Patent Application Laid-open No. 2007-123742, a ground line just under a signal line is partially removed to achieve impedance matching at the connection portion. In a technique described in Japanese Patent Application Laid-open No. 2010-212617, inductance components occurring at high frequencies are reduced by devising a wiring pattern to improve reflection and pass characteristics. In the structure described in Japanese Patent Application Laid-open No. 2010-212617, a wiring portion for connecting a transmission line portion and a terminal portion of an FPC board to each other is formed wider than a line width of the transmission line portion to increase the capacitance, thereby reducing an influence of the inductance occurring at a connection portion between a printed circuit board and the FPC board.
However, the technique described in Japanese Patent Application Laid-open No. 2010-212617 does not provide a shape that reduces the inductance of the terminal portion itself, which is the connection portion between the printed circuit board and the FPC board. Accordingly, the inductance is increased in a high-speed signal at 40 Gbps or the like and reflection is increased, resulting in increase in jitter in a signal passing through an optical module.
To solve these problems, the following measures (1) and (2) have been adopted. (1) To increase a signal pad width in the FPC board. (2) To reduce a distance between pads.
When the signal pad width is increased to about 0.8 millimeter, a connection with low reflection of −20 decibels or lower is obtained until at 40 gigahertz. However, considering positional misalignment from an ideal position occurring when the FPC board is soldered to the printed circuit board, the pad distance and the pad width of the FPC board need to be in a size that causes no short circuit with adjacent pads. For these reasons, the pad pitch and the pad width have to be set about 0.8 millimeter and 0.4 millimeter, respectively, as described in Japanese Patent Application Laid-open No. 2010-212617, and thus the inductance reduction measures such as increasing the pad width and reducing the pad distance are difficult to adopt.